[WebView] Fix the run_tests.sh script and add documentation.

android_webview/tools/system_webview_shell/test/data/run_tests_rebaseline.sh

+#!/bin/bash
+
+# Copyright 2017 The Chromium Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+# How to use:
+#
+# 1. For the tests to run an appropriate version of webview
+#    needs to be installed on the device (e.g. system_webview_apk).
+# 2. Build system_webview_shell_layout_test_apk. This will also
+#    create a base script for running tests used here.
+# 3. Execute run_test_rebaseline.sh [builddir]
+#    "builddir" is the build output directory (e.g. out/Debug/
+#    which is also the default if no directory is provided).
+#    This script will produce a shadow test_rebaseline/ directory
+#    with the new rebased expectation files.
+#    It is recommended to run this script from it's local
+#    directory in order to have the rebaseline files placed
+#    at the same level as the original test files.
+#
+#    Note: to simply run the tests without the rebaseline
+#    use the bin/run_system_webview_shell_layout_test_apk script
+#    located in the build output directory.
+#
+
+if [ "$1" == "" ]; then
+  # use the default
+  SCRIPT_BUILD_DIR="../../../../../out/Debug/"
+else
+  SCRIPT_BUILD_DIR="$1"
+fi
+
+PACKAGE_NAME="org.chromium.webview_shell.test"
+DEVICE_WEBVIEW_TEST_PATH="/sdcard/chromium_tests_root/android_webview/tools/"
+DEVICE_WEBVIEW_TEST_PATH+="system_webview_shell/test/data/"
+RUNNER=$SCRIPT_BUILD_DIR"bin/run_system_webview_shell_layout_test_apk"
+
+echo $SCRIPT_BUILD_DIR
+
+$RUNNER
+
+adb shell am instrument -w -e mode rebaseline -e class \
+    $PACKAGE_NAME.WebViewLayoutTest \
+    $PACKAGE_NAME/$PACKAGE_NAME.WebViewLayoutTestRunner
+adb pull $DEVICE_WEBVIEW_TEST_PATH ../test_rebaseline/
+
+exit 0